In recent years, in high speed and high density signal transmission between electronic devices and between printed wiring boards, transmission through conventional electric wirings is restricted in a rise in a speed and a density due to barriers brought about by mutual interference and attenuation of signals. In order to break through such restrictions, investigated is a technology of connecting electronic devices and printed wiring boards with each other by means of light, so-called optical interconnection. Polymer optical waveguides attract attentions as a transmission path of light from the viewpoints of easy processing, a low cost, a high freedom in wiring and a possible rise in a density.
A type in which the polymer optical waveguide is prepared on a glass epoxy resin substrate assuming that it is applied to optical and electrical mixed boards and a flexible type which does not have a hard support substrate assuming connection between boards are considered to be suitable as the form of the polymer optical waveguide.
A high heat resistance in addition to a high transparency (low optical transmission loss) is required to a polymer optical waveguide from the viewpoints of use environment of appliances to which it is applied and mounting of parts. (Meth)acryl polymers are known as a material for the above optical waveguide material, and optical waveguides prepared by using the above polymer are proposed (refer to, for example, patent documents 1 and 2).
However, a transparency and a heat resistance of the optical waveguides described in the patent document 1 are not specifically described and therefore are not clearly specified. Further, the optical waveguide described in the patent document 2 has a high transparency of an optical transmission loss of 0.3 dB/cm in a wavelength of 850 nm, but specific test results of an optical transmission loss, for example, after a solder reflow test are not described, and the heat resistance is not clearly specified.
On the other hand, a phenoxy resin is known as an optical waveguide material having a high heat resistance (refer to, for example, a patent document 3).
In general, a phenoxy resin is produced by one-stage method in which bisphenol A and epichlorohydrin are subjected to polycondensation reaction and by two-stage method in which an epoxy resin having a low molecular weight and bisphenol A are subjected to polyaddition reaction. It contains a lot of aromatic rings in the resin and therefore has a high heat resistance, and on the other hand, it has a high refractive index. It is indispensable to control a refractive index in designing optical parts, and when conventional phenoxy resins are used, involved therein is the problem that they have a high refractive index and therefore can not generally be used for low refractive index optical parts.
Further, a resin composition containing a carboxylic acid-modified phenoxy resin is known as a light-sensitive resin composition which is excellent in a heat resistance and which can be developed by an alkali aqueous solution (refer to, for example, patent documents 4 and 5).
However, the above resin compositions have been invented for plating resists in a production process for printed wiring boards, and they are excellent in a heat resistance but low in a transparency and are not suitable as a resin composition for an optical material.    Patent document 1: JP 6-258537 A    Patent document 2: JP 2003-195079 A    Patent document 3: JP 4-122714 A    Patent document 4: JP 2000-47383 A    Patent document 5: JP 2000-147765 A